1. Field of the Invention
The present invention relates to pressure vessels and, more specifically, to apparatus and methods for reinforcing pressure vessels. Examples of such pressure vessels include rocket motors, gas generators, and the like.
2. Description of the Related Art
It is desirable in the design and construction of certain pressure vessels, for example, high performance pressure vessels for rocketry or gas generator applications, to effectively reduce or minimize the inert mass of the vessel while maintaining or enhancing its strength and reliability. It also has been desirable to do so while containing or reducing the cost of such vessels.
Many pressure vessel designs include one or more doubly curved surfaces, for example, such as those associated with integrally formed end domes. These doubly curved surfaces may assume a number of different specific geometries or geometric cross sections. Pressure vessels used as motor casings for rocket motors, for example, typically include a cylindrical section, a domed fore section or domed fore and aft sections, and a doubly curved transition or mating section disposed between the cylindrical section and the domed section or sections. Other vessel designs also include such doubly curved surfaces, for example, such as spheres, ellipsoids, and the like.
It is generally desirable in pressure vessel design to address weak points in the vessel structure so that the vessel has a predictable, consistent, repeatable failure mode and location. By predetermining this failure mode and location, the designer may ensure that the vessel is constructed such that the operating demand of the vessel will not exceed its strength at this weakest point. In a cylindrical vessel with end domes, for example, the desired failure location often preferably is in the cylindrical section. In more weight-optimized high performance vessels, the failure location may tend to move into the end dome region, for example, due to the stress peaking from bending.
In many applications, the doubly curved surface is the area, or an area, of the vessel that is most prone to mechanical failure. This can occur for a number of reasons. As a vessel, e.g., a rocket motor, is pressurized, it experiences a number of internal stresses. One such stress is attributable to the outward force exerted from the pressurized fluid in the interior of the vessel. Depending upon the application, another such stress may be the bending moments that are exerted, particularly at the doubly curved surface, associated with relatively opposed or noncooperating forces exerted on the components of the vessel. The stress peak may be especially severe at the junction between the doubly curved surface and a nondoubly curved surface, for example, such as the junction between a hemispherical end dome and a cylindrical wall. The vessel design ideally is such that it can accommodate these bending stresses at their peaks.
Pressure vessels may be made in a number of different ways. A typical manufacturing technique used to produce solid fuel rocket motors, for example, as described in U.S. Pat. No. 4,118,262, involves the use of helically wound fiber-based composites. A material made of a high strength, continuous reinforcing filament, such as graphite or aramid fiber, for example, is impregnated with a compatible resin for hardening. The filament is wound around a mandrel, the resin is set or hardened, and the mandrel is withdrawn from the casing. The fibrous material is stabilized on the mandrel by winding around pegs in the mandrel that define holes in the fibrous material. The angle between the axis of the fiber and the longitudinal axis of the casing typically is kept as small as possible to cause forces applied to the fibers to be substantially longitudinal tensile forces. The motor shell is subsequently filled with an insulating layer, a binding layer, and a solid fuel. The filament may be woven into strips or deposited as tow. According to U.S. Pat. No. 5,348,603 to Yorgasen, for example, similar manufacturing processes may be used to wind a motor casing that is inserted into a metal shell and expanded through use of a bladder in the mandrel prior to hardening of the resin. The casing adheres to the inside of the metal shell.
In view of the design approaches used in constructing pressure vessels and the tendency for mechanical failure associated with the doubly curved surface in high performance lightweight vessel applications, it is often desirable to strengthen this doubly curved surface. One approach to providing such added strength is to reinforce that portion of the vessel by applying a reinforcing structure or material at the exterior of the vessel. A reinforcing structure, for example, may be placed over the doubly curved surface and end dome. To provide a specific example, a filament-wound helical layer could be placed over the doubly curved surface, e.g., over the end dome and transition section of a cylindrical vessel with dome, to serve as a reinforcing structure. These reinforcing structures normally would be placed upon a pressure vessel after the initial vessel has been formed, e.g., by helical winding on the mandrel. The portion or portions of the reinforcing structure that are not required for vessel reinforcement then can be removed to limit the added weight penalties created by this approach.
This approach is generally limited or disadvantageous, for example, in that it tends to be labor intensive. It also can add unnecessarily to the cost of the reinforced vessel, for example, in that the reinforcing material, which often involves considerable expense, typically is discarded and cannot readily be reused. This approach also can be limited or disadvantageous, for example, in that, as noted above, the weight of the vessel often is increased unnecessarily, especially where filament winding is used as the reinforcing material. Filament winding also can result in increased thickness at or around the polar openings of the vessel, where present, for cylindrical vessels.